Title:

Toner and image forming method

Document Type and Number:

United States Patent 6447969

Abstract:

A toner is formed of toner particles each comprising a binder resin and iron oxide particles dispersed therein. The toner particles are characterized by uniform but non-surface-exposed dispersion of the iron oxide particles within the toner particles as represented by (i) a carbon content (A) and an iron content (B) giving a ratio B/A<0.001 at surfaces of the toner particles as measured by X-ray photoelectron spectroscopy, (ii) an average circularity of at least 0.970, and (iii) at least 50% by number of toner particles satisfying D/C≦0.02, wherein C denotes a projection area-equivalent circular diameter of each toner particle and D denotes a minimum distance of iron oxide particles from a surface of the toner particle, based on a sectional view of the toner particle as observed through a transmission electron microscope (TEM).

Inventors:

Ito, Masanori (Numazu, JP)
Kukimoto, Tsutomu (Yokohama, JP)
Takiguchi, Tsuyoshi (Shizuoka-ken, JP)
Chiba, Tatsuhiko (Kamakura, JP)
Magome, Michihisa (Shizuoka-ken, JP)
Hashimoto, Akira (Mishima, JP)
Komoto, Keiji (Numazu, JP)

Plaque It!

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to a toner and an image forming method used in a recording method utilizing electrophotography, electrostatic recording, magnetic recording, toner jet recording, etc. More particularly, the present invention relates to a toner used in an image forming method for an image forming apparatus, such as a copying apparatus, wherein a toner image is once formed on an electrostatic image-bearing member and then transferred onto a transfer-receiving material to form an image thereon, and an image forming method using the toner.

Hitherto, a large number of electrophotographic processes have been known. Generally, in these processes, an electrostatic latent image is formed on an electrostatic image-bearing member (hereinafter represented by a “photosensitive member”) utilizing ordinarily a photoconductive material, the latent image is then developed with a toner to form a visible toner image, and the toner image, after being transferred as desired onto a transfer-receiving material such as paper, is fixed onto the transfer-receiving material by application of pressure, heat, etc., to provide a product copy or print. As a method for visualizing the electrostatic latent image, there have been known the cascade developing method, the magnetic brush developing method, the jumping developing method, the pressure developing method, etc.

U.S. Pat. No. 3,909,258 has proposed a developing method using a magnetic toner having an electroconductivity. More specifically, in the developing method, an electroconductive magnetic toner carried on a hollow cylindrical electroconductive sleeve with a magnet installed inside thereof is caused to contact an electrostatic image to develop the image. In this instance, at the developing region, an electroconductive path is formed of the toner particles between the electrostatic image-bearing member and the sleeve surface, and the toner particles are supplied with a charge via the electroconductive path, whereby the toner particles are attached to the electrostatic image based on a Coulomb force acting between the charge and the electrostatic image. The developing method using an electroconductive magnetic toner is an excellent method obviating problems accompanying the conventional two-component developing method, but as the toner is electroconductive, the method is accompanied with a difficulty in electrostatically transferring the developed toner image from the electrostatic image-bearing member to a transfer-receiving material (or recording material) such as plain paper.

As a developing method using a high-resistivity magnetic toner allowing electrostatic transfer, one utilizing dielectric polarization of toner particles is known. Such a developing method however essentially involves problems, such as slow developing speed and insufficient developed image density, so that the commercialization is difficult.

As another developing method using a high-resistivity insulating magnetic toner, there is known a method wherein toner particles are triboelectrically charged through friction between individual toner particles and between toner particles and a friction member such as a sleeve, and the thus-charged toner particles are caused to contact an electrostatic image-bearing member to effect a development. This method is however accompanied with a problem that the triboelectric charge is liable to be insufficient due to few opportunities of contact between the toner particles and the friction member and much magnetic material exposed to the surfaces of the magnetic toner particles, leading to inferior images due to the insufficient charge.

As another developing method, Japanese Laid-Open Patent Application (JP-A) 54-43027 and JP-A 55-18656, for example, disclose a so-called jumping developing method wherein a magnetic developer (toner) is applied in a thin layer on a developer-carrying member to be triboelectrically charged thereon, and the charged layer of the magnetic toner is moved under the action of a magnetic field to be opposed in close proximity to but free of contact with an electrostatic latent image to effect a development. According to this method,the magnetic developer is allowed to be sufficiently triboelectrically charged by application in a thin layer on the developer-carrying member, and the developer carried under a magnetic force is used for development in a state free from contact with the electrostatic latent image, so that a high definition image can be obtained with suppression of so-called “fog” caused by transfer of the developer onto non-image parts.

Such a mono-component developing method, does not require carrier particles, such as glass beads or iron powder, so that a developing device therefor can be small-sized and light in weight. Further, while the two-component developing scheme requires devices for detecting a toner concentration in the developer and for replenishing a necessary amount of toner based on the detected result in order to keep a constant toner concentration in the developer, the mono-component developing scheme does not require such devices, thus allowing a small-sized and light developing device also from these points.

However, the developing method using an insulating magnetic toner involves an unstable factor a ttributable to the use of the insulating magnetic toner. This arises from the feature that a substantial amount of fine powdery magnetic material is contained in dispersion within the insulating magnetic toner particles and a portion of the magnetic material is exposed to the toner particle surfaces to affect the flowability and the triboelectric chargeability of the magnetic toner, thereby causing a change or deterioration of properties required of the magnetic toner, such as developing performance and continuous image forming performance.

The above-mentioned problems accompanying the use of a conventional magnetic toner containing a magnetic material is considered to be principally caused by the exposure of a magnetic material to the magnetic toner particle surface. More specifically, as a result of exposure of fine particles of magnetic material having a lower resistivity than a toner biner principally constituting the toner to the toner article surfaces, various difficulties are caused, such as a lowering in toner chargeability, a lowering in toner flowability, and developer deteriorations during a long term of use, such as peeling-off of the magnetic particles due to friction between individual toner particles and toner particles and the regulating member resulting in image density lowering and occurrence of density irregularity called “sleeve ghost”.

Hitherto, various proposals have been made regarding magnetic iron oxide contained in magnetic toners, but room for improvement has yet been left.

For example, JP-A 62-279352 has proposed a magnetic toner containing silicon-containing magnetic iron oxide. The magnetic iron oxide is intentionally caused to contain silicon inside thereof, but the magnetic toner containing the magnetic iron oxide has left room for improvement regarding the flowability.

Japanese Patent Publication (JP-B) 3-9045 has proposed to provide magnetic iron oxide particles with a controlled spherical shape by adding a silicate salt thereto. The magnetic iron oxide particles obtained according to this proposal are caused to contain much silicon at an inner portion thereof and little silicon at the surface due to the use of a silicate salt for particle shape control and have a high surface smoothness. As a result, the resultant magnetic toner is provided with an improved flowability to some extent, but the adhesion between the toner binder resin and the magnetic iron oxide particles is liable to be insufficient.

JP-A 61-34070 has proposed a process for producing triiron tetroxide by adding a hydroxysilicate salt solution during oxidation to triiron tetroxide. The triiron tetroxide particles produced by the process contain Si in proximity to the surfaces thereof but are also caused to have a layer of Si in proximity to the surface thereof, so that the surface thereof is weak against a mechanical impact as by abrasion.

On the other hand, a toner has been conventionally produced through a (pulverization) process wherein a binder resin, a colorant (inclusive of a magnetic material in the case of a magnetic toner), etc., are melt-mixed for uniform dispersion, and then the mixture is pulverized by a pulverizer, and classified into toner particles having a prescribed particle size. This process however poses a restriction in material selection for complying with a recent trend for requiring a smaller particle size toner. For example, the resin-colorant dispersion mixture has to be sufficiently fragile so as to allow pulverization by a commercially feasible apparatus. If the resin-colorant dispersion mixture is sufficiently fragile for complying with the requirement, a practical high-speed pulverization of the resin-colorant dispersion mixture is liable to result in toner particles of a broad particle size range, particularly including a relatively large proportion of fine particle fraction (over-pulverized particles). Further, a toner composed of such a highly fragile material is subject to further pulverization or powder formation in copying apparatus, etc.

Further, according to the pulverization process, it is difficult to completely uniformly disperse solid fine particles of a magnetic material or a colorant in a resin, and the insufficient dispersion can lead to increased fog or lower image density while depending on the degree of the insufficiency. Further, the pulverization process essentially causes exposure of the magnetic iron oxide particles to the toner particle surfaces, thus inevitably leaving problems regarding toner flowability or charging stability in a severe environment.

Thus, the pulverization process poses a limit in production of finer toner particles as required in higher resolution and higher image quality, and the finer toner particle production is liable to result in remarkable deterioration in uniform chargeability and flowability of the toner.

For overcoming the above-mentioned problems of the pulverization process, the toner production by a suspension polymerization process has been proposed.

A toner produced through suspension polymerization (hereinafter sometimes called a “polymerization toner”) can be easily provided with a small particle size and is excellent in flowability due to its spherical toner particle shape, thus being advantageous for complying with the requirement for higher image quality.

However, such a polymerization toner is liable to have remarkably lower flowability and chargeability when it contains a magnetic material. This is because generally hydrophillic magnetic particles are liable to be exposed to the toner particle surface. For solving the problem, it is important to modify the surface property of the magnetic material.

Regarding the surface modification of a magnetic material for improved dispersion thereof in a polymerization toner, many proposals have been made. For example, JP-A 59-200254, JP-A 59-200256, JP-A 59-200257 and JP-A 59-224102 have disclosed to treat magnetic materials with various silane coupling agents. JP-A 63-250660 has disclosed to treat silicon-containing magnetic particles with a silane coupling agent. JP-A 7-72654 has disclosed to treat magnetic iron oxide with alkyltrialkoxysilane.

By such treatment, the dispersibility of a magnetic material within a toner is improved to some extent, but uniform surface hydrophobization of a magnetic material is rather difficult. As a result, the occurrence of coalescence of magnetic particles and non-hydrophobized magnetic particles is inevitable, so that the surface modification (hydrophobization) is liable to be insufficient for achieving a good level of dispersibility in the toner.

A special toner containing magnetic particles only at a specific inner portion of particles thereof has been disclosed by JP-A 7-209904, in which, however, no reference is made to the sphericity of the toner particles.

To summarize the toner organization disclosed in JP-A 7-209904, each toner particle has a structure including a surface layer of at least a certain thickness in which no magnetic particles are present. This means that the toner particle includes a substantial surface layer portion containing no magnetic particles. In another expression, this however means that such a toner particle, when in a small average particle size of 10 μm, for example, includes only a small core volume in which magnetic particles are present, so that it is difficult to incorporate a sufficient amount of magnetic particles. Further, in case where such toner particles have a particle size distribution, a large toner particle and a small toner particle have different ratios of magnetic particle-free surface layers and thus different propositions of magnetic particles, so that the developing performance and transferability of the toner particles are different depending on the toner particle sizes, thus being liable to cause a selective development phenomenon depending on particle sizes (i.e., preferential consumption of a certain toner particle size fraction). As a result, if the toner having a certain particle size distribution is used for a long period of continual image formation, toner particles containing a larger proportion of magnetic particles and exhibiting a lower developing ability, i.e., larger toner particles, are liable to remain without being consumed for the development, thus causing lowering in image density and image quality and inferior fixability.

As for printer apparatus, laser beam printers and LED printers are becoming predominant on the market in recent years, and correspondingly, higher resolutions are being desired, e.g., from a conventional level of 240 and 300 dpi to 400, 600 and 800 dpi. For these reasons, the developing scheme is also required to be adapted for higher resolution. Further, also copying machines are required to comply with high functionality copying, and digital-mode copying apparatus are becoming predominant. Along with this trend, the latent image formation by using laser beam is predominant together with a requirement for higher resolution. Accordingly, similarly as in printers, higher resolution and higher definition developing scheme is being required. For complying with such demands, smaller particle size toners having a specific particle size distribution have been proposed in, e.g., JP-A 1-112253, JP-A 1-191156, JP-A 2-214156, JP-A 2-284158, JP-A 3-181952, and JP-A 4-162048.

On the other hand, in recent years when environmental protection is thought much of, a conventional primary charging and transfer process utilizing corona discharge is being gradually shifted to a primary charging and transfer process using a charging member abutted against an electrostatic image-bearing member.

More specifically, in the conventional primary changing and transfer process utilizing corona discharge, a substantial amount of ozone is generated at the time of corona discharge, particularly for generating negative corona, so that an image forming apparatus has to be equipped with a filter for ozone capture, which has required a larger apparatus size and an increased running cost. Such a corona charging scheme has also caused image defects, such as the so-called image flow caused by a lowering in surface resistivity of the photosensitive member due to attachment f ozone adducts, such as nitrogen oxide, and memory of the photosensitive member caused by ions remaining within the charger during the intermission of the image forming apparatus.

For solving the above-mentioned problems encountered in the corona charging system, a contact charging system or a contact transfer system has been developed, wherein a charging member or a transfer member in the form of, e.g., a roller or a blade, is caused to contact a photosensitive member surface to form a narrow space in proximity to the contact portion and cause a discharge presumably according to the Paschen's law, thereby suppressing the occurrence of ozone to the minimum, e.g., as disclosed in JP-A 57-178257, JP-A 56-104351, JP-A 58-40566, JP-A 58-139156, and JP-A 58-150975. Particularly, a charging scheme and a transfer scheme using an electro-conductive elastic roller as disclosed in JP-A 63-149669 and JP-A 2-123385 have been preferably used in view of the stability.

However, it has been also found that the contact charging system or the contact transfer system is accompanied with a problem to be considered not encountered in the corona discharge system.

More specifically, first in the contact transfer system wherein a transfer member is pressed against a photosensitive member via a transfer paper (i.e., transfer receiving material), at the time of transfer of a toner image on the photosensitive member to the transfer paper, the toner image is compressed thereby to cause a partial transfer failure so-called “hollow image” or “transfer dropout”. Further, as the toner particle size is reduced for complying with a recent demand for a higher resolution and higher definition developing scheme, the forces of attaching toner particles onto the photosensitive member (such as image force and Van der Waals force) become predominant compared with Coulomb force acting on the toner particles for transfer, whereby the transfer residual toner is liable to be increased or the transfer failure is liable to be more serious.

On the other hand, in the contact charging system wherein a charging member is pressed against a photosensitive member surface at a certain pressure, the transfer residual toner is pressed against the photosensitive member surface, so that the photosensitive member surface is liable to be abraded and the toner melt-sticking is liable to be caused at the part of abrasion as a nucleus. This tendency becomes particularly noticeable if the amount of the transfer residual toner is increased.

The occurrence of the abrasion of and toner melt-sticking onto the photosensitive member causes serious defects in electrostatic image formation on the photosensitive member. More specifically, the abrasion of photosensitive member causes a failure of primary charging, so that the part of abrasion results in a black trace in a halftone image. The toner melt-sticking causes a failure of latent image formation by exposure, the part of melt-stuck toner results in a white trace in a halftone image. Further, these defects also deteriorate the toner transferability. Accordingly, in combination with the above-mentioned transfer failure caused by the contact transfer system, remarkable image defects are liable to occur, and the image quality deterioration can be accelerated synergistically in some cases.

The problems of the photosensitive member abrasion and transfer failure are liable to occur especially in the case of using a toner comprising indefinitely-shaped or non-spherical toner particles. This is presumably because of a lower transferability of the non-spherical toner particles and the presence of toner particle edges liable to scratch the photosensitive member surface. Further, the abrasion problem becomes severer in the case of using magnetic toner particles containing a magnetic material exposed to the surface thereof. This may be easily understood in view of a state that the exposed magnetic particles are directly pressed against the photosensitive member.

Further, when the amount of transfer residual toner is increased, it becomes difficult to retain sufficient contact between the contact charging member and the photosensitive member, so that the charging performance is lowered, thus being liable to cause a transfer of toner to non-image portion, i.e., fog in the case of reversal development. This difficulty is liable to be encountered in a low humidity environment wherein the resistivity of the charging member is increased.

As described above, in the image forming system including the contact charging system and the contact transfer system which are very preferable from an ecological viewpoint, it is desirable to develop and use a magnetic toner exhibiting high transferability and less liable to cause photosensitive member abrasion and toner melt-sticking.

On the other hand, in the case where some transfer residual toner remains after a transfer step of transferring a toner image formed on a photosensitive member in the developing step to a transfer-receiving material, the transfer residual toner has to be cleaned and recovered in a waste toner vessel in a cleaning step. In the cleaning step, a cleaning blade, a cleaning fur brush or a cleaning roller has been conventionally used. Any cleaning means has relied on mechanically scraping off or damming the transfer residual toner for recovery into the waste toner vessel. However, the use of such a mechanical cleaning means wears and shortens the life of the photosensitive member. From the apparatus viewpoint, the presence of cleaning device has posed an obstacle to provision of a compact apparatus. Further, from the viewpoints of ecology and effective toner utilization, a system free from generation of waste toner, i.e., a cleanerless system, is desirable.

Such cleanerless image forming systems have been discussed in JP-A 59-133573, JP-A 62-203182, JP-A 63-133179, JP-A 64-20587, JP-A 2-302772, JP-A 5-2289, JP-A 5-53482 and JP-A 5-61383. Moreover, a serious attention has not been paid to a desirable toner organization to be used in such cleanerless image forming systems.

JP-A 61-279864 has proposed a toner having specific shape factors SF-1 and SF-2, no reference is made to a transfer step using the toner. Further, as a trace experiment of ours, the toner exhibited a toner efficiency which is low and therefore has left a room for improvement.

JP-A 63-235953 has disclosed a magnetic toner sphered by mechanical impact, but the transfer efficiency thereof is still low and has left a room for further improvement.

Incidentally, a cleanerless image forming system including a simultaneous developing and cleaning scheme, a photosensitive member surface is rubbed with a toner and a toner-carrying member for recovering a toner on a non-image portion and supplying a toner to an image portion on the photosensitive member by the toner-carrying member. At the time of rubbing, if reversibly charged toners inclusive of transfer residual toner and fog toner can be oppositely charged to a normal polarity, such toners can be potentially easily recovered.

As a result of our study, in case where a conventional toner containing a magnetic material is used in such an image forming system including a simultaneous developing and cleaning scheme, a partial electrical continuity is caused at the time of developing between the photosensitive member and the tone-carrying member via the toner due to the magnetic material exposed to the toner particle surface, so that the electrostatic latent image on the photosensitive member is disturbed thereby and it is difficult to obtain a high definition image. Further, such a magnetic toner containing a magnetic material exposed to the toner particle surface causes an insufficient charge of the transfer residual toner, so that the smooth recovery thereof from the photosensitive member during the developing step is obstructed. Further, at the time of rubbing of the photosensitive member with the toner and the toner-carrying member, the photosensitive member is liable to be severely worn due to the magnetic material exposed to toner particle surface, thus shortening the life of the photosensitive member. As a result, there results in a so-called ghost image, i.e., a soiling toner images attached onto a non-image region.

Accordingly, in an image forming system including a simultaneous developing and cleaning scheme, a magnetic material-containing toner is desired to be free from exposure of the magnetic material to the toner particle surface.

Further, in an image forming system retaining a cleaning member while including a simultaneous developing and cleaning scheme, if the abutting pressing of the cleaning member against the photosensitive member is lowered in order to retain a longer life of the photosensitive member, an increased amount of the transfer residual toner can slip by the cleaning member to reach the developing step. In such a system, it is also very important to minimize the amount of the transfer residual toner stipping by the cleaning member even under a reduced abutting pressure of the cleaning member.

The above-mentioned problems encountered in the case of using a conventional magnetic material-containing magnetic toner have been principally caused by the exposure of the magnetic material to the toner particle surface. As another factor, in the case of a magnetic toner containing a magnetic material exposed to the toner particle surface, the magnetic toner is liable to have an unstable chargeability in a high humidity environment due to a lower resistivity of the magnetic material than the toner binder resin, thus causing difficulties, such as increased fog, lower transferability and a lower recovery rate of the transfer residual toner leading to the occurrence of ghost images, in addition to the performance deterioration of the photosensitive member due to abrasion of the photosensitive member by rubbing with the exposed magnetic material.

In view of the above factors, a magnetic toner exhibiting good initial performances and stability of performances in an image forming system including a simultaneous developing and cleaning scheme has not been obtained as yet.

SUMMARY OF THE INVENTION

A generic object of the present invention is to provide a toner and an image forming method having solved the above-mentioned problems of the prior art.

A more specific object of the present invention is to provide a magnetic toner which exhibits stable chargeability, less susceptible of environmental changes, and can provide images having high image density and with suppressed fog at a good image reproducibility even after a long period of continual use.

Another object of the present invention is to provide an image forming method which has solved the above-mentioned problems in the image forming process based on the contact development-scheme capable of omitting a cleaner system and can provide images free from fog and ghost with excellent resolution, transferability and excellent durability without being affected by environmental conditions.

Another object of the present invention is to provide an image forming method including a contact changing step of less ozone-generation type and a non-contact developing method using a magnetic toner (mono-component developer) providing images with less fog, wherein a magnetic toner exhibiting good transferability to cause less transfer dropout and less transfer residual toner and less abrading the photosensitive member, thus being less liable to result in image defects even after a long period of continual use.

Another object of the present invention is to provide an image forming method capable of stable electrostatic latent image formation even in a low humidity environment and resulting in less image defects such as fog due to a lowering in chargeability in continuous image formation.

According to the present invention, there is provided a toner, comprising: toner particles each comprising at least a binder resin and iron oxide particles, wherein

(i) the toner particles exhibit a carbon content (A) and an iron content (B) giving a ratio B/A<0.001 at surfaces of the toner particles as measured by X-ray photoelectron spectroscopy,

(ii) the toner particles exhibit an average circularity of at least 0.970, and

(iii) the toner particles contain at least 50% by number of toner particles satisfying D/C≦0.02, wherein C denotes a projection area-equivalent circular diameter of each toner particle and D denotes a minimum distance of iron oxide particles from a surface of the toner particle, based on a sectional view of the toner particle as observed through a transmission electron microscope (TEM).

According to another aspect of the present invention, there is provided an image forming method, comprising:

a charging step of charging an electrostatic image-bearing member with a charging member receiving a voltage from an external voltage supply,

an exposure step of exposing the electrostatic image-bearing member to form an electrostatic latent image thereon,

a developing step of developing the electrostatic latent image with the above-mentioned toner carried on a toner-carrying member to form a toner image on the electrostatic image-bearing member, and

a transfer step of transferring the toner image onto a transfer-receiving material.

These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an image forming apparatus adopting a non-contact developing scheme.

FIG. 2 is an enlarged view around a developing device included in the image forming apparatus of FIG. 1.

FIG. 3 is a schematic illustration of an image forming apparatus adopting a contact developing scheme.

FIG. 4 is an enlarged view around a developing device included in the image forming apparatus of FIG. 3.

FIG. 5 is a schematic illustration of a contact transfer member.

FIG. 6 is a waveform diagram showing an example of developing bias voltage waveform.

FIG. 7 illustrates a checker pattern for evaluating a toner developing performance.

FIG. 8 is a schematic partial sectional view of a photosensitive member showing its layer structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As a result of our study regarding the uniformity and stability of chargeability of a magnetic toner, it has been found very effective to provide a magnetic toner satisfying the following properties (i) and (ii) in combination for realizing the uniform and stable chargeability.

(i) the toner particles exhibit a carbon content (A) and an iron content (B) giving a ratio B/A<0.001 at surfaces of the toner particles as measured by X-ray photoelectron spectroscopy,

(ii) the toner particles exhibit an average circularity of at least 0.970, based on the following definition of circularity ø for each toner particle:

circularity ø=L₀/L,

wherein L₀denotes a peripheral length of a circle having an area equal to the projection area of the particle, and L denotes a peripheral length of the projection image of the particle.

Moreover, it has been found possible by using the magnetic toner to remarkably suppress the abrasion of a photosensitive member, insufficient charging, and transfer failure and stably provide high definition images free from image defects such as fog in a long period of use even in an image forming method including no cleaner but adopting a contact charging scheme wherein ghost images are liable to occur due to toner recovery failure, or an image forming method including a contact charging step, a monocomponent non-contact developing step and a contact transfer step.

The above properties (i) and (ii) have not been satisfied by conventional magnetic toners containing magnetic iron oxide. As a result of our detailed study, it has been discovered that the dissatisfaction of the properties is caused by failure in sufficient and uniform hydrophobization of magnetic iron oxide before inclusion into a magnetic toner.

In preparation of a magnetic toner, the dispersibility of magnetic iron oxide particles in a toner binder resin can be improved by using the magnetic iron oxide particles after surface hydrophobization. Further, even if a substantial amount of the magnetic iron oxide is exposed to the toner particle surfaces, the chargeability of the toner is less impaired in any environment, if the surface of the exposed magnetic iron oxide has been uniformly hydrophobized. This per se has been well known.

Accordingly, prior to the present invention, various methods for surface hydrophobization of magnetic iron oxide particles have been proposed. According to the methods proposed heretofore, however, it has not been easy to obtain magnetic iron oxide particles which have been sufficiently and uniformly hydrophobized. A higher hydrophobicity can be attained if a larger amount of hydrophobizing agent or a hydrophobizing agent having a higher viscosity is used. In this case, however, the coalescence of magnetic iron oxide fine particles is likely to occur, so that better hydrophobicity and better dispersion has not been necessarily achieved in combination.

Further to say, untreated magnetic iron oxide surface is generally hydrophillic, it is necessary to hydrophobize such a hydrophillic iron oxide in order to obtain a hydrophobic iron oxide. According to surface treating methods proposed heretofore, the uniformity of the resultant hydrophobicity has been insufficient, a conventional magnetic toner using such hydrophobized magnetic iron oxide is caused to have a chargeability which varies depending on humidity, etc., and is not sufficiently stable.

In contrast thereto, the iron oxide used as a magnetic material in the toner of the present invention has been provided with a very high level of uniform hydrophobicity. This is for example achieved by effecting a hydrophobization surface treatment while causing hydrolysis of a coupling agent (i.e., hydrophobizing agent) in an aqueous medium wherein magnetic iron oxide particles are dispersed in primary particles. Compared with gaseous phase treatment, such hydrophobization treatment in an aqueous medium is less liable to cause coalescence of magnetic iron oxide particles so that the iron oxide can be surface-treated in a substantially primary particle state, thereby achieving the hydrophobization at a high uniformity level.

Moreover, the method of surface treating iron oxide while causing hydrolysis of a coupling agent does not necessitate the use of a gas-generating coupling agent, such as chlorosilanes and silazanes, but allows the use of a high-viscosity coupling agent which has been difficult to use in a gaseous phase treatment because it is liable to cause the coalescence of magnetic iron oxide particles.

The coupling agents usable in the present invention may include, for example, silane coupling agents and titanate coupling agents. Silane coupling agents are preferred, as represented by a general formula of R_mSiY_n, wherein R denotes an alkoxy group; m denotes an integer of 1-3; Y denotes a hydrocarbon group such as alkyl, vinyl, glycidoxy or methacryl; and n denotes an integer of 1-3. Specific examples thereof may include: vinyltrimethoxysilane, vinyltriethoxysilane, γ-methacryloxypropyl-trimethoxysilane, vinyltriacetoxysilane, methyltrimethoxysilane, methyltriethoxysilane, isobutyltrimethoxysilane, dimethyldimethoxysilane, dimethyldiethoxysilane, trimethylmethoxysilane, hydroxypropyltrimethoxysilane, phenyltrimethoxysilane, n-hexadecyltrimethoxysilane, and n-octadecyltrimethoxysilane.

It is particularly prefered to use an alkyltrialkoxysilane coupling agent represented by the following formula for hydrophobizing iron oxide in an aqueous medium.

C_pH_2p+1—Si(OC_qH_2q+1 )₃,

wherein p denotes an integer of 2-20, and q denotes an integer of 1-3.

In the above formula, if p is smaller than 2, the hydrophobization treatment becomes easier, but it becomes difficult to impart a sufficient hydrophobicity. On the other hand, if p is larger than 20, a sufficient hydrophobicity can be imparted, but the coalescence of iron oxide particles is liable to occur so that it becomes difficult to disperse the treated iron oxide particles in the toner.

Further, if q is larger than 3, the silane coupling agent is caused to have a lower reactivity, so that sufficient hydrophobization becomes difficult.

It is particularly preferred to use an alkyltrialkoxysilane represented by the above formula wherein p is an integer of 2-20, more preferably 3-15, and q is an integer of 1-3, more preferably 1 or 2.

The coupling agent may preferably be used for treatment in an amount of 0.05-20 wt. parts, more preferably 0.1-10 wt. parts, per 100 wt. parts of iron oxide.

The aqueous medium used for the hydrophobization treatment in the present invention refers to a dispersion medium principally comprising water. Specific examples of the aqueous medium may include: water per se, a mixture of water with a minor amount of surfactant, water containing a pH controlling agent, and a mixture of water with an organic solvent. The surfactant may preferably be a nonionic surfactant, such as polyvinyl alcohol. The surfactant may be added in 0.1-5 wt. % in water. The pH controlling agent may for example be an inorganic acid, such as hydrochloric acid.

The hydrophobization treatment may preferably be performed under sufficient stirring so as to disperse the iron oxide particles in particles within the aqueous medium, e.g., by means of a mixer having stirring blades, preferably a high-shearing force mixer, such as Attritor and TK-Homomixer.

The thus-treated iron oxide particles have been uniformly surface-hydrophobized and therefore can be very well dispersed in the toner binder resin, thus providing toner particles of which the surface is free from exposure of the iron oxide particles. As a result of using such treated iron oxide, it becomes possible to obtain the toner of the present invention characterized by the feature (i) that the toner particles exhibit a carbon content (A) and an iron content (B) giving a ratio B/A<0.001 at surfaces of the toner particles as measured by X-ray photoelectron spectroscopy whereby the toner is provided with uniform and stable chargeability for achieving high-quality image forming performances and highly stable continuous image forming performances. If the ratio B/A is below 0.0005, the uniform and stable chargeability is further improved.

More specifically, the iron oxide used in the present invention may for example be produced through a process as described belows.

To a ferrous salt aqueous solution, an alkali, such as sodium hydroxide, in an amount equivalent to the iron in the ferrous salt or larger to prepare an aqueous solution containing ferrous hydroxide. While retaining the pH of the thus-prepared aqueous solution at pH 7, preferably pH 8-10 and warming the aqueous solution at a temperature of 70° C. or higher, air is blown into the aqueous solution to oxidize the ferrous hydroxide, thereby first forming seed crystals functioning as nuclei of magnetic iron oxide particles to be produced.

Then, to the slurry-form liquid containing the seed crystals, an aqueous solution containing ferrous salt in an amount of ca. 1 equivalent based on the amount of the previously added alkali, is added. While keeping the liquid at pH 6-10, air is blown thereinto to proceed with the reaction of the ferrous hydroxide, thereby growing magnetic iron oxide particles around the seed crystals as nuclei. Along with the progress of the oxidation reaction, the liquid pH is shifted toward an acidic side, but it is preferred not to allow the liquid pH go down to below 6. At a final stage of the oxidation, the liquid pH is adjusted, and the slurry liquid is sufficiently stirred so as to disperse the magnetic iron oxide in primary particles. In this state, a coupling agent for hydrophobization is added to the liquid to be sufficiently mixed under stirring. Thereafter, the slurry is filtered out and dried, and the dried product is lightly disintegrated to provide hydrophobic treated magnetic iron oxide particles. Alternatively, the iron oxide particles after the oxidation reaction may be washed, filtered out and then, without being dried, re-dispersed in another aqueous medium. Then, the pH of the re-dispersion liquid is adjusted and subjected to hydrophobization by adding a coupling agent under sufficient stirring.

Magnetic iron oxide particles are produced by adding an alkali to an aqueous ferrous salt solution, oxidizing the ferrous salt at an elevated temperature, and further adding an aqueous ferrous salt solution.

Anyway, it is important that untreated iron oxide particles formed in the oxidation reaction system is subjected to hydrophobization in its wet slurry state and without being dried prior to the hydrophobization. This is because if the untreated iron oxide particles are dried as they are, the primary particles thereof are inevitably coalesced or agglomerated to some extent. It is difficult or substantially impossible to effect uniform hydrophobization of magnetic iron oxide particles, if such partially coalesced or agglomerated magnetic iron oxide particles are subjected to a hydrophobization treatment even in a wet system, thus failing to provide uniformly hydrophobized magnetic iron oxide particles giving toner particles satisfying B/A<0.001, as a characteristic of the toner according to the present invention.

As the ferrous salt used in the above-mentioned production process, it is generally possible to use ferrous sulfate by-produced in the sulfuric acid process for titanium production or ferrous sulfate by-produced during surface washing of steel sheets. It is also possible to use ferrous chloride.

In the above-mentioned process for producing magnetic iron oxide from a ferrous salt aqueous solution, a ferrous salt concentration of 0.5-2 mol/liter is generally used so as to obviate an excessive viscosity increase accompanying the reaction and in view of the solubility of a ferrous salt, particularly of ferrous sulfate. A lower ferrous salt concentration generally tends to provide finer magnetic iron oxide particles. Further, as for the reaction conditions, a higher rate of air supply, and a lower reaction temperature, tend to provide finer product particles.

By using the thus-produced hydrophobic magnetic iron oxide particles for toner production, it becomes possible to obtain the toner exhibiting excellent image forming performances and stability according to the present invention.

Incidentally, JP-B 60-3181 discloses a process for producing a magnetic polymerization toner containing magnetic particles which have been hydrophobized by surface treatment with a silane coupling agent in a wet system. However, the wet surface treatment with a silane coupling agent is applied to dry powdery untreated magnetic particles. Such dry magnetic fine particles have inevitably caused coalescence of particles by agglomeration during the drying step, so that uniform hydrophobization of individual magnetic particles is difficult even by a wet-system surface treatment. Even if a polymerization toner is produced by using such surface-treated magnetic particles, it is difficult to achieve a ratio B/A<0.001, a characteristic of the toner according to the present invention.

It is another important feature of the toner according to the present invention that (iii) the toner particles contain at least 50% by number of toner particles satisfying D/C≦0.02, wherein C denotes a projection area-equivalent circular diameter of each toner particle and D denotes a minimum distance of iron oxide particles from a surface of the toner particle, based on a sectional view of the toner particle as observed through a transmission electron microscope (TEM). It is further preferred that the toner particles contain 65% or more by number, more preferably 75% or more by number, of toner particles satisfying the relationship of D/C≦0.02.

If below 50% by number of toner particles satisfy the relationship of D/C≦0.02, this means that a major proportion of toner particles contain no ion oxide particles in a superficial region outside the boundary line defined by D/C=0.02. If such a toner particle is assumed to have a true spherical region, the iron oxide-free superficial region occupies at least 11.5% by volume of the toner particle. In an actual toner particle, the iron oxide particles inside the superficial region do not form a core region uniformly packed with the iron oxide particles, so that the iron oxide-free superficial region for such a toner particle clearly occupies more than 12% by volume. A toner composed of a major proportion of such toner particles having a substantial volume of iron oxide-free superficial region is therefore accompanied with several difficulties as mentioned before, such as incapability of incorporating a sufficient amount of iron oxide particles and a larger difference in developing and transfer performances depending on toner particle sizes.

The D/C values discussed herein are based on values measured in the following manner. Sample toner particles are sufficiently dispersed in a cold-setting epoxy resin, which is then hardened at 40° C. for 2 days. The hardened product is sliced, as it is or a further frozen state, into thin flakes by a microtome having a diamond cutter.

The thus-obtained thin flakes are photographed at a magnification of 1×10⁴through a transmission electron microscope (TEM) (Model “H-600”, available from Hitachi K.K.) under an acceleration voltage of 100 kV. On the thus-taken photographs, toner particles having provided sectional views (areas) giving a circle-equivalent diameter (C) falling within a range of ±10% of the number-average particle size (determined according to the Coulter counter method described hereinafter) of the sample toner particles are taken for determination of a minimum distance (D) of iron oxide particles (having a particle size of at least 0.03 μm) in a toner particle from the surface of the toner particle to calculate a value D/C of the toner particle. From the measured values of D/C for a statistically sufficient number of toner particle sectional views, the percentage by number of toner particles giving D/C≦0.02 is determined for the sample toner particles.

A toner satisfying (i) B/A<0.001 and (iii) at least 50% by number of toner particles satisfying D/C≦0.02, means a toner which is free from localization of the iron oxide at the toner particle surface and also free from extreme localization of the iron oxide at the core, i.e., a toner comprising toner particles wherein the iron oxide is substantially uniformly dispersed but the surface exposure thereof is effectively suppressed. These requirements (i) and (iii) of the present invention cannot be satisfied by a non-uniform distribution of the iron oxide in the toner particles.

If the toner according to the present invention comprising toner particles substantially free from the surface exposure of iron oxide particles (i.e., satisfying (i) B/A<0.001), the surface abrasion of a photosensitive member is substantially prevented, and the surface abrasion and toner sticking onto the photosensitive member can be remarkably reduced over a long period of operation even in an image forming system wherein the toner is pressed against a photosensitive member by a charging member, a transfer member, etc.

The above-mentioned hydrophobized iron oxide may preferably be used in amount of 10-200 wt. parts, more preferably 20-180 wt. parts, per 100 wt. parts of the binder resin in the toner of the present invention. If the iron oxide is below 10 wt. parts, the coloring power of the toner is liable to be insufficient, and the suppression of fog becomes difficult. On the other hand, above 200 wt. parts, the toner is held onto the toner-carrying member under an excessively large magnetic holding force to exhibit a lower developing performance. Moreover, the uniform dispersion of the iron oxide particles in toner particles becomes difficult, and the fixability is liable to be lowered.

The toner according to the present invention is also characterize by a specifically high circularity. In order to reduce the toner attachment onto a non-image part and the transfer residual toner on the photosensitive member, it is necessary that the toner particles are charged sufficiently and uniformly. Further, in the case of using a small particle size toner exhibiting a large attachment force of toner particles from the viewpoint of higher image quality. The toner particle shape also greatly affects the toner attachment force onto the non-image part. More specifically, if toner particles have shapes which are closer to a sphere and more uniform, the toner particles are caused to have smaller attachment areas, thus reducing the amounts of toner attached not the non-image part and transfer residual toner on the photosensitive member, thus achieving higher image quality and stabler continuous image forming performances.

In view of these factors, the toner according to the present invention is required to have an average circularity of at least 0.97 for achieving the high image quality and high stability.

As a result, the toner of the present invention exhibits a reduced toner attachment force. Because of the reduced attachment force and the above-mentioned stable chargeability, the toner according to the present invention exhibits a remarkably improved efficiency of transfer from the photosensitive member to a transfer-receiving material, such as paper. This is an important toner performance for achieving a high resolution in addition to a minute dot image reproducibility.

Accordingly, by using a spherical toner according to the present invention, the amount of transfer residual toner is remarkably reduced. As a result, even in an image forming material including a contact charging step, the amount of toner present at an abutting part between the charging member and the photosensitive member is reduced, so that the abrasion of and toner melt-sticking onto the photosensitive member may be prevented to remarkably reduce image defects corresponding thereto. Further, toner particles exhibiting an average circularity of 0.970 or higher according to the present invention are substantially free from surface edge parts, so that they do not substantially scratch the photosensitive member surface even if they are present at the abutting position between the charging member and the photosensitive member, thereby suppressing the abrasion of the photosensitive member surface. These effects can also be remarkably exhibited in an image forming method including a contact transfer step wherein transfer dropout is liable to occur.

The toner according to the present invention may preferably have a weight-average particle size (D4) of 2-10 μm. Above 10 μm, the reproducibility of minute dot images is physically lowered so that the toner charge stability in a severe environment according to the present invention cannot be fully utilized. On the other hand, below 2 μm, the toner flowability is liable to be lowered, even if the other features of the toner according to the present invention, such as sphericity and surface non-exposure of the iron oxide, are relied on so that difficulties such as fog and lower density are liable to occur due to the charging failure.

Thus, the toner according to the present invention can exhibit remarkable improvements as in charging stability and flowability over the conventional toners, in case where it has a weight-average particle size (D4) of 2-10 μm, preferably 3-10 μm, more preferably 3.5-8.0 μm for further high image quality.

Examples of polymerizable monomers constituting a polymerizable monomer mixture may include: styrene monomers, such as styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, p-methoxystyrene and p-ethylstyrene; acrylate esters, such as methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, n-propyl acrylate, n-octyl acrylate, dodecyl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, 2-chloroethyl acrylate and phenyl acrylate; methacrylate esters, such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-octyl methacrylate, dodecyl methacrylate, 2-ethylhexyl methacrylate, stearyl methacrylate, phenyl methacrylate, dimethylaminoethyl methacrylate, and diethylaminoethyl methacrylate; acrylonitrile, methacrylonitrile and acrylamide. These monomers may be used singly or in mixture. Among these, styrene or a styrene derivative may preferably be used singly or in mixture with another monomer so as to provide a toner with good developing performances and continuous image forming performances.

In a preferred embodiment, the toner according to the present invention may contain 0.5-50 wt. % of a release agent. Ordinarily, a toner image formed on a photosensitive member is transferred onto a transfer-receiving material in a transfer step, and the toner image is then fixed onto the transfer-receiving material under application of an energy, such as heat, pressure, etc., to provide a semipermanent image. For the fixation, a hot roller fixation scheme is frequently used. As mentioned above, a toner having a weight-average particle size of at most 10 μm can provide a very high definition image, but such fine toner particles when transferred onto paper as a transfer-receiving material are liable to enter gaps between paper fibers, thus receiving insufficient heat energy from the heat-fixation roller to cause low-temperature offset. By incorporating an appropriate amount of wax as a release agent in the toner of the present invention, it becomes possible to effectively prevent the abrasion of the photosensitive member while satisfying high resolution and anti-offset property in combination.

Examples of the wax usable in the toner according to the present invention may include: petroleum waxes, such as paraffin wax, microcrystalline wax and petrolatum, and derivatives thereof; montan wax and derivatives thereof, hydrocarbon wax obtained through Fischer-Tropsche process and derivatives thereof, polyolefin waxes as represented by polyethylene wax and derivatives thereof, and natural waxes such as carnauba wax and candellila wax and derivatives thereof. The derivatives herein may include: oxides, block copolymers and graft-modified products with vinyl monomers. It is also possible to use higher aliphatic alcohols, aliphatic acids such as stearic acid and palmitic acid and derivatives thereof, acid amide wax, ester wax, ketone, hardened castor oil and derivatives thereof, negative waxes and animal waxes.

Among these waxes, those providing a DSC curve on temperature increase (as measured by using a differential scanning calorimeter) showing a maximum heat-absorption peak in a range of 40-110° C., particularly 45-90° C., are preferred. A wax satisfying the above feature may effectively develop releasability while remarkably improving the low-temperature fixability. If the maximum heat-absorption peak appears at below 40° C., the wax exhibits only weak self cohesion, thus resulting in inferior anti-high temperature offset property. On the other hand, if the maximum heat-absorption peak appears at above 110° C., the fixation temperature becomes high and low-temperature offset is liable to occur. In the case of polymerization toner production in an aqueous medium, if the maximum heat-absorption temperature is high, the wax is liable to precipitate during dispersion of the polymerizable monomer mixture containing the wax in the aqueous medium.

The DSC measurement for determining the maximum heat-absorption peak temperature of a wax component may be performed according to ASTM D3418-8 by using, e.g., “DSC-7” available from Perkin-Elmer Corp. Temperature compensation of the detector unit may be performed based on melting points of indium and zinc, and caloric calibration may be made based on the fusion heat of indium. For measurement, a sample is placed on an aluminum pan and heated at a rate of 10° C./min. together with a blank pan as a control.

The wax component may preferably be contained in 0.5-50 wt. % of the binder resin. Below 0.5 wt. %, the low-temperature offset suppression effect is scarce. Above 50 wt. %, the long-term storability of the toner is lowered, and the dispersibility of other toner ingredients is lowered to result in inferior toner flowability and lower image forming performances.

In preparation of the toner of the present invention by polymerization, it is possible to incorporate a resin in the monomer mixture. For example, in order to introduce a polymer having a hydrophillic functional group, such as amino, carboxyl, hydroxyl, sulfonic acid, glicidyl or nitrile, of which the monomer is unsuitable to be used in an aqueous suspension system because of its water-solubility resulting in emulsion polymerization, such a polymer unit may be incorporated in the monomer mixture in the form of a copolymer (random, block or graft-copolymer) of the monomer with another vinyl monomer, such as styrene or ethylene; or a polycondensate, such as polyester or polyamide; or polyaddition-type polymer, such as polyether or polyimine. If a polymer having such a polar functional group is included in the monomer mixture to be incorporated in the product toner particles, the phase separation of the wax is promoted to enhance the encapsulation of the wax, thus providing a toner with better anti-offset property, anti-blocking property, and low-temperature fixability. Such a polar polymer may preferably be used in 1-20 wt. parts per 100 wt. parts of the polymerizable monomer. Below 1 wt. part, the addition effect is scarce, and above 20 wt. parts, the physical property designing of the resultant polymerization toner becomes difficult. The polymer having such a polar functional group may preferably have an average molecular weight of at least 3000. Below 3000, particularly below 2000, the polymer is excessively concentrated at the surface of the product toner particles to adversely affect the developing performance and anti-blocking property of the toner. On the other hand, if a polymer having a molecular weight different from the molecular weight range of the polymer resulting from polymerization of the monomer(s) is incorporated in the monomer mixture, the resultant toner may be provided with a broader molecular weight distribution favoring a higher anti-offset property.

The toner according to the present invention may contain a charge control agent for acquiring a stable chargeability. The charge control agent may be known one but may preferably be one providing a high charging speed and stably providing a constant charge.

Further, in the case of toner production through the polymerization process, it is particularly preferred to use a charge control agent which exhibits little polymerization inhibition effect and contains substantially no fraction soluble in the aqueous dispersion medium. Specific examples of such negative control agents may include: metal compounds of aromatic carboxylic acids, hydroxycarboxylic acids or dicarboxylic acids, such as salicylic acid, alkylsalicylic acids, dialkylsalicylic acid and naphthoic acid; metal salts or metal complexes of azo dyes or azo pigments; polymeric compounds having sulfonic acid or carboxylic acid group in their side chains, boron compounds, urea compounds, silicon compounds and calix arenes. Examples of positive charge control agents may include: quaternary ammonium salts, polymeric compounds having such a quaternary ammonium salt group in their side chains, guanidine compounds, nigrosine compounds and imidazole compounds. Such a charge control agent may preferably be contained in 0.5-10 wt. parts per 100 wt. parts of the binder resin. However, the inclusion of a charge control agent is not essential to the toner of the present invention. For example, the inclusion of a charge control agent can be omitted, if the toner is used in an image forming method wherein triboelectrification by friction with a toner layer regulation member or a toner-carrying member is positively used.

The iron oxide used as a magnetic material in the toner of the present invention may principally comprise triiron tetroxide or γ-iron oxide optionally containing one or more elements, such as phosphorus, cobalt, nickel, copper, magnesium, manganese, aluminum or silicon. The iron oxide particles may preferably have a BET specific surface area of 2-30 m²/g, more preferably 3-28 m²/g, and a Moh's hardness of 5-7.

The iron oxide particles may have octahedral, hexahedral, spherical, acicular or flaky shape, but iron oxide particles having less anisotropic shapes, such as octahedral, hexahedral, spherical or indefinite shape are preferred in order to provide a high image density. Such particle shapes may be confirmed by observation through a scanning electron microscope (SEM). It is preferred that the iron oxide particles have a volume-average particle size of 0.1-0.3 μm and contain at most 40% by number of particles of 0.03-0.1 μm, based on measurement of particles having particle sizes of at least 0.03 μm.

Iron oxide particles having an average particle size of below 0.1 μm are not generally preferred because they are liable to provide a magnetic toner giving images which are somewhat tinted in red and insufficient in blackness with enhanced reddish tint in halftone images. Further, as the iron oxide particles are caused to have an increased surface area, the dispersibility thereof is lowered, and an inefficiently larger energy is consumed for the production. Further, the coloring power of the iron oxide particles can be lowered to result in insufficient image density in some cases.

On the other hand, if the iron oxide particles have an average particle size in excess of 0.3 μm, the weight per one particle is increased to increase the probability of exposure thereof to the toner particle surface due to a specific gravity difference with the binder during the production. Further, the wearing of the production apparatus can be promoted and the dispersion thereof is liable to become unstable.

Further, if particles of 0.1 um or smaller exceed 40% by number of total particles (having particle sizes of 0.03 um or larger), the iron oxide particles are liable to have a lower dispersibility because of an increased surface area, liable to form agglomerates in the toner to impair the toner chargeability, and are liable to have a lower coloring power. If the percentage is lowered to at most 30% by number, the difficulties are preferably alleviated.

Incidentally, iron oxide particles having particle sizes of below 0.03 μm receive little stress during the toner production so that the probability of exposure thereof to the toner particle surface is low. Further, even if such minute particles are exposed to the toner particle surface, they do not substantially function as leakage sites lowering the chargeability of the toner particles. Accordingly, the particles of 0.03-0.1 μm are noted herein, and the percentage by number thereof is suppressed to below a certain limit.

On the other hand, if particles of 0.3 μm or larger exceed 10% by number, the iron oxide particles are caused to have a lower coloring power, thus being liable to result in a lower image density. Further, as the number of iron oxide particles is decreased at an identical weight percentage, it becomes difficult statistically to have the iron oxide particles be present up to the proximity of the toner particle surface and distribute equal numbers of iron oxide particles to respective toner particles. This is undesirable. It is further preferred that the percentage be suppressed to at most 5% by number.

In the present invention, it is preferred that the iron oxide production conditions are adjusted so as to satisfy the above-mentioned conditions for the particle size distribution, or the produced iron oxide particles are used for the toner production after adjusting the particle size distribution as by pulverization and/or classification. The classification may suitably be performed by utilizing sedimentation as by a centrifuge or a thickener, or wet classification using, e.g., a cyclone.

The volume-average particle size and particle size distribution of iron oxide particles described herein are based on values measured in the following manner.

Sample particles in a sufficiently dispersed state are photographed at a magnification of 3×10⁴through a transmission electron microscope (TEM), and 100 particles each having a particle size of at least 0.03 μm selected at random in visual fields of the taken photographs are subjected to measurement of projection areas. The particle size (projection area-equivalent circle diameter) of each particle is determined as a diameter of a circle having an area equal to the measured projection area of the particle. Based on the measured particle sizes of the 100 particles, a volume-average particle size, percentage by number of particles of 0.03 μm-0.1 μm and percentage by number of particles of 0.3 μm or larger are determined. Identical determination can also be made automatically by using an image analyzer.

The volume-average particle size and particle size distribution of iron oxide particles dispersed within toner particles may be measured in the following manner.

Sample toner particles are sufficiently dispersed in a cold-setting epoxy resin, which is then hardened for 2 days at 40° C. The hardened product is sliced into thin flakes by a microtome. The thin flakes are observed through a TEM and photographic at magnification of 1×10⁴-4×10⁴. One hundred iron oxide particles of at least 0.03 μm in particle size selected at random in visual fields of the taken photographs are subjected to measurement of projection areas. From the projection areas of the 100 iron oxide particles, a volume-average particle size (projection area-equivalent circular diameter), percentage by number of particles of 0.03 μm-0.1 μm and percentage by number of particles of 0.3 μm or larger are determined similarly as the above.

The toner of the present invention can also contain another colorant in addition to the magnetic iron oxide. Examples of such another colorant may include: magnetic or non-magnetic inorganic compounds and known dyes and pigments. Specific examples thereof may include: particles of ferromagnetic metals, such as cobalt and nickel, alloys of these metals with chromium, manganese, copper, zinc, aluminum and rare earth elements, hematite, titanium black, nigrosine dye/pigment, carbon black and phthalocyanine. Such another colorant can also be surface-treated.

For the preparation of a polymerization toner, a polymerization initiator exhibiting a halflife of 0.5-30 hours at the polymerization temperature may be added in an amount of 0.5-20 wt. % of the polymerizable monomer so as to obtain a polymer exhibiting a maximum in a molecular weight range of 1×10⁴-1×10⁵, thereby providing the toner with a desirable strength and appropriate melt-characteristics. Examples of the polymerization initiator may include: azo- or diazo-type polymerization initiators, such as 2,2′-azobis-(2,4-dimethylvaleronitrile), 2,2′-azobisisobutyronitrile, 1,1′-azobis(cyclohexane-2-carbonitrile), 2,2′-azobis-4-methoxy-2,4-dimethylvaleronitrile, azobis-isobutyronitrile; and peroxide-type polymerization initiators such as benzoyl peroxide, methyl ethyl ketone peroxide, diisopropyl peroxycarbonate, cumene hydroperoxide, 2,4-dichlorobenzoyl peroxide, and lauroyl peroxide.

The polymerizable monomer mixture can further contain a crosslinking agent in a proportion of preferably 0.001-15 wt. % of the polymerizable monomer.

In the toner production by suspension polymerization, a polymerizable monomer mixture is formed by mixing the polymerizable monomer and the iron oxide with other toner ingredients, as desired, such as a colorant, a release agent, a plasticizer, another polymer and a crosslinking agent, and further adding thereto other additives, such as an organic solvent for lowering the viscosity of the polymer produced in the polymerization, a dispersing agent, etc. The thus-obtained polymerizable monomer mixture is further subjected to uniform dissolution or dispersion by a dispersing means, such as a homogenizer, a ball mill, a colloid mill or an ultrasonic disperser, and then charged into and suspended in an aqueous medium containing a dispersion stabilizer. In this instance, if the suspension system is subjected to dispersion into a desired toner size without a break by using a high-speed dispersing machine, such as a high-speed stirrer or an ultrasonic disperser, the resultant toner particles are provided with a sharper particle size distribution. The polymerization initiator may be added to the polymerizable monomer together with other ingredients as described above or immediately before suspension into the aqueous medium. Alternatively, it is also possible to add the polymerization initiator as a solution thereof in the polymerizable monomer or a solvent to the suspension system immediately before the initiation of the polymerization.

After the particle or droplet formation by suspension in the above-described manner using a high-speed dispersion means, the system is stirred by an ordinary stirring device so as to retain the dispersed particle state and prevent the floating or sedimentation of the particles.

In the suspension polymerization process, a known surfactant, or organic or inorganic dispersant, may be used as the dispersion stabilizer. Among these, an inorganic dispersant may preferably be used because it is less liable to result in deleterious ultrafine powder, the resultant dispersion stability is less liable to be broken even at a reaction temperature change because the dispersion stabilization effect is attained by its stearic hindrance, and it is easily washed to be free from leaving adverse effect to the toner. Examples of the inorganic dispersant may include: polyvalent metal phosphates, such as calcium phosphate, magnesium phosphate, aluminum phosphate and zinc phosphate; carbonates, such as calcium carbonate and magnesium carbonate; inorganic salts, such as calcium metasilicate, calcium sulfate and barium sulfate; and inorganic oxides, such as calcium hydroxide, magnesium hydroxide, aluminum hydroxide, silica, bentonite and alumina.

These inorganic dispersant may be used singly or in combination of two or more species in 0.2-20 wt. parts per 100 wt. parts of the polymerizable monomer. In order to obtain toner particles having a further small average size of, e.g., at most 5 μm, it is also possible to use 0.001-0.1 wt. part of a surfactant in combination.

Examples of the surfactant may include: sodium dodecylbenzene sulfate, sodium tetradecyl sulfate, sodium pentadecyl sulfate, sodium octyl sulfate, sodium oleate, sodium laurate, sodium stearate, and potassium stearate.

Such an inorganic dispersant as described above may be used in a commercially available state as it is, but in order to obtain fine particles thereof, such an inorganic dispersant may be produced in an aqueous medium prior to dispersion of the polymerizable monomer mixture in the aqueous system. For example, in the case of calcium phosphate, sodium phosphate aqueous solution and calcium aqueous chloride aqueous solution may be blended under high-speed stirring to form water-insoluble calcium phosphate allowing more uniform and finer dispersion. At this time, water-soluble sodium chloride is by-produced, but the presence of a water-soluble salt is effective for suppressing the dissolution of a polymerizable monomer in the aqueous medium, thus suppressing the production of ultrafine toner particles due to emulsion polymerization, and thus being more convenient. The presence of a water-soluble salt however can obstruct the removal of the residual polymerizable monomer in the final stage of polymerization, so that it is advisable to exchange the aqueous medium or effect desalting with ion-exchange resin. The inorganic dispersant can be removed substantially completely by dissolution with acid or alkali after the polymerization.

In the polymerization step, the polymerization temperature may be set to at least 40° C., generally in the range of 50-90° C. By polymerization in this temperature range, the release agent or wax to be enclosed inside the toner particles may be precipitated by phase separation to allow a more complete enclosure. In order to consume a remaining portion of the polymerizable monomer, the reaction temperature may possibly be raised up to 90-150° C. in the final stage of polymerization.

The toner particles of the present invention may preferably be blended with inorganic fine powder or hydrophobized inorganic fine powder as a flowability-improving agent to provide the toner according to the present invention. Examples thereof may include: titanium oxide fine powder, silica fine powder and calcium fine powder. Silica-fine powder is particularly preferred.

Such inorganic fine powder may preferably exhibit a specific surface area of at least 30 m²/g, particularly 50-400 m²/g, as measured by the BET method according to nitrogen adsorption, so as to provide good results.

The silica fine powder used in the present invention may comprise either the dry-process silica or fumed silica formed by vapor-phase oxidation of silicon halides, or wet-process silica as produced from water glass. It is however preferred to use the dry-process silica accompanied with less surface or integral silanol group and with less production residue.

The silica fine powder used in the present invention should preferably be a hydrophobized one. The hydrophobization may be performed by chemically treating silica fine powder with an organic silicon compound, etc., reacting with or being adsorbed by the silica fine powder. As a preferred method, a dry-process silica fine powder formed by vapor-phase oxidation of a silicon halide may be treated with a silane coupling agent, and then or simultaneously therewith, treated with an organosilicon compound, such as silicone oil.

Examples of the silane coupling agent may include: hexamethyldisilazane, trimethylsilane, trimethylchlorosilane, trimthylethoxysilane, dimethyldichlorosilane, methyltrichlorosilane, allyldimethylchlorosilane, allylphenyldichlorosilane, benzyldimethylchlorosilane, bromomethyldimethylchlorosilane, α-chloroethyltrichlorislane, triorganosilane mercaptan, trimethylsilyl mercaptan, triorganosilyl acrylate, vinyldimethylacetoxysilane, dimethylethoxysilane, dimethyldimethoxysilane, diphenyldiethoxysilane, hexamethyldisiloxane, 1,3-divinyltetramthyldisiloxane, and 1,3-diphenyltetramethyldisiloxane.

As the organosilicon compound, silicone oil may be used. Silicone oil preferably used may have a viscosity of ca. 30-1000 mm²/s (cSt). Preferred examples thereof may include: dimethylsilicone oil, methylphenylsilicone oil, α-ethylstyrene-modified silicone oil, chlorophenylsilicone oil, and fluorine-containing silicone oil.

The treatment with a silicone oil may be performed by mixing base silica fine powder (already treated with or to be treated simultaneously with a silane coupling agent) with the silicone oil directly in a blender, such as a Henschel mixer, or spraying the silicone oil onto the base silica fine powder. Alternatively, it is also possible to apply a method wherein the silicone oil is dissolved or dispersed in an appropriate solvent, and the base silica fine powder is mixed therewith, followed by removal of the solvent.

The toner according to the present invention can further contain external additives other than the flowability improver, as desired.

For example, in order to improve the cleanability, it is possible to further add fine particles having a primary particle size exceeding 30 nm (and preferably also specific surface area of below 50 m²/g), more preferably close-to-spherical inorganic or organic fine particles having a primary particle size of at least 50 nm (and preferably also a specific surface area of below 30 m²/g), as a preferred mode. For example, it is preferred to use spherical silica particles, spherical polymethylsilsesquioxane particles or spherical resin particles.

Examples of other external additives may include: lubricant powder, such as polytetrafluoroethylene powder, zinc stearate powder, and polyvinylidene fluoride powder; abrasives, such as cerium oxide powder, silicon carbide powder and strontium titanate powder; anti-caking agents; and electroconductivity-imparting agents, such as carbon black powder, zinc oxide powder, and tin oxide powder. It is also possible to add a minor amount of opposite-polarity organic fine particles or inorganic fine particles as a developing improver. It is possible that these additives have been surface-hydrophobized.

The above-mentioned external additive, may be added in a proportion of 0.1-5 wt. parts, preferably 0.1-3 wt. parts, per 10 wt. parts of the toner.

In the case of producing the toner of the present invention through a pulverization process, a known process may be adopted. For example, essential ingredients of the toner including the binder resin, the iron oxide, a release agent, a charge control agent, and optionally, a colorant, and other additives, may be sufficiently blended in a mixing means, such as a Henschel mixer or a ball mill, and then melt-kneaded by a hot heating means, such as hot rollers, a kneader or an extruder, to melt-mixing the resins and disperse or dissolve other ingredients including the iron oxide in the resin. After cooling, the melt-kneaded product is pulverized, classified and optionally surface-treated to obtain toner particles, which are then blended with external additives such as a flowability improver to obtain the toner according to the present invention. The classification and the surface treatment can be performed in this order or in a reverse order. The classification may preferably be performed by using a multi-division classifier in view of the production efficiency.

The pulverization may be performed by using known pulverizing apparatus of the mechanical impact type or the jetting type. In order to attain a specific circularity of the toner of the present invention, it is preferred to effect the pulverization under heating or apply a supplementary mechanical impact. It is also possible to subject the toner particles after pulverization (and optionally further classification) to dispersion in a hot water bath or passage through a hot gas stream.

The application of a mechanical impact may be effected by using, e.g., “Kryptron” system (available from Kawasaki Jukogyo K.K.) or “Turbo Mill” (available from Turbo Kogyo K.K.). It is also possible to use a system wherein toner particles are directed toward a casing inner wall by blades rotating at a high speed so as apply a mechanical impact as by compression and friction to the toner particles, such as “Mechano-Fusion” system (available from Hosokawa Micron K.K.) or “Hybridization” system (available from Nara Kikai Seisakusho K.K.).

In the case of applying a mechanical impact as a surface treatment, the environment temperature for the treatment may preferably be set in the neighborhood of the glass transition point Tg of the toner (i.e., in a range of Tg±30° C.) from the viewpoint of prevention of agglomeration and productivity. The treatment in the temperature range of Tg±20° C. is further preferred so as to particularly effectively increase the transfer efficiency.

It is also possible to produce the toner of the present invention according to a method of using a disk or a multi-fluid nozzle for spraying the melt-mixture into the air to form spherical toner particles as disclosed in JP-B 56-13945; a method of directly producing toner particles through polymerization in an aqueous organic solvent wherein the monomer is soluble but the resultant polymer is insoluble; or an emulsion polymerization method as represented by a soap-free polymerization wherein toner particles are directly produced by polymerization in the presence of a water-soluble polymerization initiator.

Examples of the binder resin for producing the toner according to the present invention through the pulverization process may include: homopolymers of styrene and its substitution derivatives, such as polystyrene and polyvinyltoluene; styrene copolymers, such as styrene-propylene copolymer, styrene-vinyltoluene copolymer, styrene-vinylnaphthalene copolymer, styrene-methyl acrylate copolymer, styrene-ethyl acrylate copolymer, styrene-butyl acrylate copolymer, styrene-octyl acrylate copolymer, styrene-dimethylaminoethyl acrylate copolymer, styrene-methyl methacrylate copolymer, styrene-ethyl methacrylate copolymer, styrene-butyl methacrylate copolymer, styrene-dimethylaminoethyl methacrylate copolymer, styrene-vinyl methyl ether copolymer, styrene-vinyl ethyl ether copolymer, styrene-vinyl methyl ketone copolymer, styrene-butadiene copolymer, styrene-isoprene copolymer, styrene-maleic acid copolymer, and styrene-maleic acid ester copolymer; polymethyl methacrylate, polybutyl methacrylate, polyvinyl acetate, polyethylene, polypropylene, polyvinyl butyral, silicone resin, polyester resin, polyamide resin, epoxy resin, polyacrylic acid resin, rosin, modified rosin, terpene resin, phenolic resin, aliphatic or alicyclic hydrocarbon resin, aromatic petroleum resin, paraffin wax, and carnauba resin. These resins may be used singly or in mixture of two or more species. Styrene copolymers and polyester resins are particularly preferred in view of developing performances and fixability.

Next, a developing method using the toner according to the present invention will be described, first with respect to a system wherein a photo-sensitive member (an electrostatic image-bearing member) and a toner-carrying member do not contact with each other (as illustrated in FIGS. 1 and 2).

In such a non-contact developing system, a magnetic toner is applied in a toner-carrying member in a layer thickness smaller than a closest gap between the toner-carrying member and a photosensitive member to effect a development under application of an alternating bias electric field. Such a thin toner layer may be formed by using a toner layer thickness regulation member disposed above the toner-carrying member. In a preferred embodiment, an elastic toner layer thickness regulating means is abutted against the toner carrying member so as to uniformly charge the magnetic toner.

The toner-carrying member may preferably be disposed opposite to the photosensitive member with a spacing therefrom of 100-500 μm, more preferably 120-500 μm. Below 100 μm, the toner developing performance can be remarkably changed due to a fluctuation in spacing, so that it becomes difficult to produce image forming apparatus exhibiting stable image forming performances in a large scale. Above 500 μm, the followability of the toner onto a latent image on the photosensitive member is lowered to result in lower image qualities, such as a lower resolution and a lower image density. Moreover, in the case of a simultaneously developing and cleaning system, the efficiency of recovery of transfer residual toner is lowered to result in foggy images due to toner recovery failure.

The toner layer may preferably be formed at a rate of 5-30 g/m²on the toner-carrying member. Below 5 g/m², it becomes difficult to attain a sufficient image density, and because of excessive toner charge, the toner layer is liable to be accompanied with a coating irregularity. Above 30 g/m², toner scattering is liable to be caused.

The toner carrying member may preferably have a surface roughness Ra (JIS centerline-average roughness) in the range of 0.2-3.5 μm. If Ra is below 0.2 μm, the toner on the toner-carrying member is liable to be excessively charged, thus exhibiting insufficient developing performance. Above 3.5 μm, the toner layer on the toner-carrying member is liable to cause coating irregularity, thus resulting in density irregularity on the resultant images. The surface roughness may further preferably be in the range of 0.5-3.0 μm.

The surface roughness Ra of the toner carrying member refers to a center line-average roughness as measured by a surface roughness tester (“SURFCODER SE-30H”, available from K.K. Kosaka Kenkyusho) according to JIS B0601. More specifically, the surface roughness Ra may be determined by taking a measurement length a of 2.5 mm along a center line (taken on an x-axis) and taking a roughness on a y-axis direction to represent the roughness curve by a function of y=f(x) to calculate a surface roughness Ra (μm) from the following equation:

Ra=(1/a)∫₀^a |f(x)|dx.< /p>

The magnetic toner according to the present invention has a high chargeability, so that a total charge thereof should preferably be controlled at the time of development. Accordingly, the toner-carrying member may preferably be surface-coated with a layer of resin in which electroconductive fine particles and/or a lubricant is dispersed.

The electroconductive fine particles contained in the coating resin layer on the toner-carrying member may preferably comprise one species or a combination of two or more species selected from carbon black, graphite, and electroconductive metal oxides or metal complex oxides, such as electroconductive zinc oxide. The coating resin for dispersion of the electroconductive fine particles and/or lubricant may comprise a known resin, such as phenolic resin, epoxy resin, polyamide resin, polyester resin, polycarbonate resin, polyolefine resin, silicone resin, fluorine-containing resin, styrene resin, or acrylic resin. A thermosetting resin or photosetting resin is particularly preferred.

In the non-contact developing method, the moving speed (surface speed) of the toner-carrying member carrying and conveying the toner thereon may preferably be different from that of the photosensitive member in the developing region. By providing such a moving speed difference, the toner particles can be sufficiently supplied from the toner-carrying member to the photosensitive member, thus providing good images.

The toner-carrying member surface can move in an identical direction or a reverse direction with respect to the surface moving direction of the photosensitive member, preferably at a relative speed of 1.02-3.0 times.

The development is performed by transferring the magnetic toner under application of an alternating bias electric field onto an electrostatic latent image. The alternating bias electric field may preferably comprise a peak-to-peak electric field intensity of 3×10⁶-1×10⁷V/m and a frequency of 100-500 Hz. It is also preferred to superpose a DC bias electric field thereon.

Next, a system wherein a toner-carrying member and a photosensitive member (electrostatic image-bearing member) contact each other for development (as illustrated in FIGS. 3 and 4) will be described.

In such a contact developing system, a reversal development mode is preferred. It is also preferred to adopt a simultaneous developing and cleaning scheme, so as to allow a substantial reduction in size of entire apparatus. In this instance, a DC or AC bias electric field may be applied at the time of development in a blank period before or after the development so as to provide a controlled potential allowing the development and the recovery of residual toner on the photosensitive member. The DC component is set between the bright-part potential and the dark-part potential.

The toner-carrying member may preferably comprise an elastic roller, on which the toner is applied to contact the photosensitive member surface. In this instance, as the development is effected by an electric field acting between the photosensitive member and the elastic roller via the toner, it is necessary that a potential is present at the surface of or in the vicinity thereof of the elastic roller, and an electric field is formed across a narrow gap between the photosensitive member surface and the elastic roller surface. For this purpose, it is possible to use an elastic roller having an elastomer layer of a medium region of controlled resistivity to retain an electric field while preventing the continuity with the photosensitive member surface, or an electroconductive roller coated with a thin surface insulating layer. Alternatively, it is also possible to use an electroconductive resin sleeve having an insulating substance layer on its surface facing the photosensitive member or an insulating sleeve having an electroconductive layer on its surface not facing the photosensitive member. It is also possible to use a toner-carrying member in the form of a rigid roller in combination with a photosensitive member in the form of a flexible belt. The elastic roller as a toner-carrying member may preferably have a resistivity in the range of 10²-10⁹ohm.cm.

The toner-carrying member may preferably have a surface roughness Ra in the range of 0.2-3.0 μm so as to satisfy a high image quality and a high durability. If Ra exceeds 3.0 μm, the thin toner layer formation on the toner-carrying member becomes difficult, and the toner charging performance is not improved, so that in improved image quality cannot be expected. If the Ra is set to be 3.0 μm or below, the toner-conveying performance on the toner-carrying member surface is suppressed, and as a thin toner layer is formed thereon, the frequency of contact between the toner and the toner-carrying member is increased to improve the toner charging performance. As a synergy of these effects, the image quality is improved. On the other hand, if Ra is below 0.2 μm, the control of toner coating amount becomes difficult.

In the contact developing method, the toner-carrying member surface may be moved either in an identical direction or in a reverse direction with respect to the photosensitive member. In the case of movement in identical direction, the toner-carrying member may preferably be moved (or rotated) at a circumferential speed which is 1.05-3.0 times that of the photosensitive member.

If the circumferential speed of the toner-carrying member is below 1.05 times that of the photosensitive member, the toner on the photosensitive member receives an insufficient stirring effect, so that a good image quality cannot be expected. Further, in the case of developing an image requiring a large amount of toner over a wide area, such as a solid black image, the toner supply onto the electrostatic latent image is liable to be insufficient, thus resulting in a lower image density. At a higher circumferential speed ratio, the amount of the toner supply to the developing site is increased and the frequency of toner attachment to and separation from the latent image is increased to enhance the repetition of recovery from an unnecessary part and attachment onto a necessary part, thus providing an image faithful to the latent image. However, if the circumferential speed ratio exceeds 3.0, various problems (such as an image density lowering due to an excessive charge of the toner) are caused by excessive charging of the toner, and toner deterioration and the toner sticking onto the toner-carrying member due to mechanical stress are caused and promoted.

Next, a step of charging the photosensitive member will be described.

In the present invention, while a non-contact charging step as by using a corona charger can be adopted, it is preferred to adopt a contact charging scheme wherein a charging member is abutted to the photosensitive member for charging the photosensitive member. In this instance, a charging roller may preferably be used as a contact charging member.

The charging roller may preferably be operated under at a roller abutting pressure of 4.9-490 N/m (5-500 g/cm) under application of a DC voltage or a DC voltage superposed with an AC voltage. In the case of DC/AC superposed voltage, it is preferred that the AC voltage=0.5-5 kVpp, AC frequency=50 Hz to 5 kHz and DC voltage=±0.2 to ±5 kV.

As another contact charging means, it is possible to use a charging blade or a charging brush. By using such contact charging means, the charging voltage can be substantially lowered, and the ozone generation can be suppressed.

The charging roller and the charging blade as the contact charging means may preferably comprise electroconductive rubber and can be surface-coated with a releasable film. The releasable film may comprise, e.g., nylon-based resin, PVdF (polyvinylidene fluoride), PVdC (polyvinylide

3909258	Electrographic development process	Kotz
4108786	Magnetic dry developer for electrostatic photography and process for preparation thereof	Takayama et al.	252/62.1
4380384	Charging device for electronic copier	Ueno et al.
4530894	Coated magnetic toner powder	Imamura et al.	430/106.6
4620987	Surface treatment method for magnetic particles	Yamashita et al.	427/131
4664504	Image forming apparatus	Oda et al.
4727395	Reverse developing image forming apparatus with small drum	Oda et al.
4769676	Image forming apparatus including means for removing residual toner	Mukai et al.
4820603	Magnetic toner	Sakashita	430/106.6
4843424	Reverse developing image forming apparatus with disturbing means	Oda et al.
4851960	Charging device	Nakamura et al.	361/225
4904562	Process for producing encapsulated toner	Yusa et al.	430/106.6
4957840	Developer and image forming device	Sakashita et al.	430/106.6
4992191	Sphere-like magnetite particles and a process for producing the same	Mori et al.	252/62.59
5014089	Developer in an image forming device having a binding resin and magnetic powder	Sakashita et al.
5137796	Magnetic developer, comprising spherical particles magnetic	Takiguchi et al.	430/106.6
5194359	Developing method for one component developer	Kanbe et al.	430/120
5262267	Magnetic developer, image forming method and image forming apparatus	Takiguchi et al.	430/122
5282007	Cleanerless image forming method	Oshiumi
5310615	Image forming method	Tanikawa	430/106.6
5450180	Image forming apparatus having constant current and voltage control in the charging and transfer regions	Ohzeki et al.
5672454	Toner containing particulate magnetic materials	Sasaki et al.	430/106.6
5721433	Apparatus and method for analyzing particle images including measuring at a plurality of capturing magnifications	Kosaka	250/573
6132919	Polymerized toner and production process thereof	Ogawa et al.	430/110

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